1. Field of the Invention
The present invention relates to a microphone unit which can be effectively mounted and used in e.g. an electrical or electronics product (hereafter referred to simply as product) such as a mobile phone or a hearing aid, and which detects sound (i.e. vibration of air) and converts the detected sound to an electrical signal for output.
2. Description of the Related Art
A microphone unit is known which has a vibratory diaphragm to be vibrated by sound so as to detect the sound by the vibration of the vibratory diaphragm. One of the microphone units of this kind is designed to guide the sound to both front and rear surfaces of the vibratory diaphragm. In such a microphone unit, the vibratory diaphragm is vibrated by the difference between the sound guided to the front surface and the sound guided to the rear surface of the vibratory diaphragm (i.e. difference in phase and amplitude of the vibration waves of air), whereby the sound is detected by the vibration of the vibratory diaphragm. Such a microphone unit is called a differential type microphone unit.
One of such known differential type microphone units has a housing and a vibratory diaphragm provided in the housing so that the inner space of the housing is divided into a first inner space (space on the front surface side of the vibratory diaphragm) and a second inner space (space on the rear surface side of the vibratory diaphragm), and that the front wall of the housing (wall facing the front surface of the vibratory diaphragm) has a first through-hole formed therein to connect the first inner space and an outer space outside the housing, while the rear wall of the housing (wall facing the rear surface of the vibratory diaphragm) has a second through-hole formed therein to connect the second inner space and the outer space outside the housing. In such a differential type microphone unit, sound is guided from the outer space through the first through-hole to the first inner space (i.e. the front surface of the vibratory diaphragm), while the sound is also guided from the outer space through the second through-hole to the second inner space (i.e. the rear surface of the vibratory diaphragm).
However, there is a problem that it is not suitable to mount and use the conventional differential type microphone unit in e.g. a product such as a mobile phone or a hearing aid. More specifically, assuming that the product has a sound receiving opening, and that the conventional microphone is mounted in the product to allow the first through-hole in the front wall of the housing of the microphone to face the sound receiving opening, there is a possibility that the second through-hole in the rear wall of the housing is occluded by some components in the product. If the second through-hole is occluded, it is not possible to effectively guide the sound to the second inner space (to the rear surface of the vibratory diaphragm), making it impossible to obtain good differential characteristics and reducing the quality of the microphone.
There are other known microphone units in the art. For example, Japanese Laid-open Patent Publication 2008-48329 discloses a microphone unit having a circuit board and a vibratory diaphragm supported by a support to the circuit board, in which the support has a leg member, while a space is formed between the vibratory diaphragm and the circuit board. Japanese Laid-open Patent Publication 2008-219238 discloses a microphone unit having a vibratory diaphragm supported by a support ring in a unit case, in which the support ring has a leg member, and a space is formed between the vibratory diaphragm and the unit case. Further, Japanese Patent 3601900 discloses a microphone unit having a housing in which a cylindrical tube-shaped rib for inserting a differential microphone is provided. However, these known microphone units do not solve the above problem.